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re: Coloring 2DPlot with different colors
*To*: mathgroup at yoda.physics.unc.edu
*Subject*: re: Coloring 2DPlot with different colors
*From*: Paul=Rubin%Management%Business at banyan.cl.msu.edu
*Date*: Mon, 13 Jun 94 11:02:10 EDT
Wang Jian-hua <WANGJH at am.nie.ac.sg> wrote:
>Does any one knows how to make on 2DPlot with different colors? For
>example, I'd like to use two different color to emphasize decreasing or
>increasing of a function or the positive and negative values of a function.
>Thanks for help.
>Jian-hua
There are at least two ways (the not-so-easy way and the not-even-that-easy
way). The easier of the two, when you are going to use a small number of
distinct colors, is to split the function you want to plot into several
functions, each taking values on a different part of the domain. For
example, to plot the Sin function with different colors for positive and
negative values:
In[]:= f1[x_] := Sin[x] /; Sin[x] >= 0
In[]:= f2[x_] := Sin[x] /; Sin[x] < 0
Function f1 fails to reduce to a number if the value of Sin[x] is negative.
In[]:= Off[ Plot::plnr ];
In[]:= Plot[ {f1[x], f2[x]}, {x, 0, 6 Pi},
PlotStyle -> {{Hue[ .3 ]}, {Hue[ .7 ]}} ];
In[]:= On[ Plot::plnr ];
For each value of x in Plot, either f1 or f2 fails to yield a number, so no
point is plotted for that function. I turned off the warning messages that
produces. I used Hue, but RGBColor, CYMKColor or GrayLevel would work just
as well.
To get closer to continuous coloring takes a lot more work. Here is the Sin
function plotted with the Hue at each point equal to the absolute value of
the ordinate:
In[]:= p = Plot[ Sin[x], {x, 0, 6 Pi},
DisplayFunction -> Identity ];
(* Store the plot in p but don't display it *)
In[]:= q = First[ First[ Cases[ p, Line[_], 20 ] ] ];
(* Isolate the argument of the Line[] command in q *)
In[]:= q // Short
-16
Out[]= {{0., 0.}, <<159>>, {18.8496, -7.34764 10 }}
(* q is a list of 161 points *)
In[]:= q1 = Partition[ q, 2, 1 ];
(* Partition the point list into a list of line segments *)
In[]:= q1 // Short
Out[]= {{{0., 0.}, {0.392699, 0.382683}}, <<159>>}
(* q1 has 160 pairs of points *)
In[]:= q2 = {Hue[ Abs[ #[[1, 2]] ] ], Line[ # ]}& /@ q1;
(* Replace each pair of points with a pair of graphics primitives, Hue
and Line, where the argument for Hue is the ordinate of the left
endpoint of the line segment *)
In[]:= q2 // Short
Out[]= {{Hue[0.], Line[{{0., 0.}, <<1>>}]}, <<159>>}
In[]:= p1 = p /. {Line[_]} -> q2;
(* Replace the line in the original plot with this list *)
In[]:= Show[ p1, DisplayFunction -> $DisplayFunction ]
(* Display the result *)
**************************************************************************
* Paul A. Rubin Phone: (517) 336-3509 *
* Department of Management Fax: (517) 336-1111 *
* Eli Broad Graduate School of Management Net: RUBIN at MSU.EDU *
* Michigan State University *
* East Lansing, MI 48824-1122 (USA) *
**************************************************************************
Mathematicians are like Frenchmen: whenever you say something to them,
they translate it into their own language, and at once it is something
entirely different. J. W. v. GOETHE
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